In the field of a silver halide photographic processes a photographic process for obtaining a positive photographic image by a single step after step after image exposure without need of reversal processing for obtaining positive images is called a direct positive photographic process, and the photographic material for use in such a photographic process is called a direct positive photographic material. Typical examples of the direct positive photographic processes are: a process for developing silver halide grains after imagewise exposing the silver halide grains in the presence of a desensitizer; and a process involving surface-developing a silver halide emulsion having light-sensitive nuclei mainly in the inside of the silver halide grains in the presence of a nucleating agent or under an overall uniform exposure (fogging by light exposure) after imagewise exposure. The present invention particularly relates to the latter process.
That is, a silver halide emulsion having light-sensitive nuclei in the inside of the silver halide grains, and hence forming a latent image mainly in the inside of the silver halide grains, is called an internal latent image-type silver halide emulsion, and is fundamentally different from an ordinary silver halide emulsion mainly forming a latent image on the surfaces of the silver halide grains.
In the internal latent image-type silver halide emulsion, the silver halide grains are not substantially surface-nucleating developed, since the latent image formed on the light-sensitive nuclei in the inside of the silver halide grains capture electrons from the nucleating agent. On the other hand, since the unexposed silver halide grains do not have an internal latent image, a latent image (fogged nuclei) is formed on the surface of the silver halide grains by the provision of electrons from the nucleating agent, whereby the unexposed silver halide grains can be surface-developed. Thus, by the surface nucleating development after exposure, a positive image is formed in one step.
In order for the aforesaid process for obtaining a direct positive image by a surface nucleating development to be practically used, improvements in the fundamental photographic characteristics, such as the improvement of photographic sensitivity, increase of the maximum density (D.sub.max), reduction of the minimum density (D.sub.min), etc., are, as a matter of course, required, and for such purposes, many patents and patent publications (including unexamined publications) as described hereinafter have been provided for the improvement of such photographic materials, such as with respect to grains of the silver halide emulsions, sensitizers, nucleating agents, etc. However, the formation of a re-reversal negative image which is a significant defect in a practical internal latent image-type directive positive silver halide emulsion, has not yet been prevented in spite of the fact that the formation of the re-reversal negative image is a significant problem reducing image quality. The formation of the re-reversal negative image specific to the internal latent image-type silver halide emulsion is remarkable when the photographic material is exposed under high illuminance, and in particular when the photographic material is exposed to sunlight or an electronic flash of a camera and causes development on the photographic material as dark black spots. Thus, a positive counterplan for preventing the formation of such a re-reversal negative image under high illuminance and for improving image quality has been strongly desired.